This invention generally relates to optical modulation. More particularly, it relates to a device for high data rate modulation of an optical signal. Even more particularly, the invention relates to a device that provides a high data rate of magneto-optical modulation.
Because of the high data rates available, optical fiber is preferred for high-speed transmission of data, audio, and video. Binary optical signals consist of low and high to intensity signals traveling through the fiber. A limiting factor in optical fiber communication networks has been the speed at which light can be electrically switched or modulated to provide change from a high intensity signal to a low intensity signal and back to a high intensity signal. This conversion from electrical to optical signal is slower than the capability of the fiber. While the optical fiber can accommodate much higher data rates, commercial techniques for creating high-speed modulation are presently achieving approximately 40 billion bits per second, or 40 GHz.
One method of modulating an optical signal involves providing a digital optical signal directly from a light source. In this method light is directly modulated by turning on and turning off power to a laser source. It is difficult, however, to make these transitions quickly without introducing non-linear effects that can degrade the signal. These effects include changes in index of refraction of material in the laser cavity which effectively changes the optical path length of the cavity during the pulse, leading to an effect called chirp, and provides greater dispersion of the signal as it travels down optical fiber.
Alternatively, a continuous wave light source can be externally modulated to provide a digital optical signal. One such method is electroabsorption modulation. Continuous wave light is directed through a semiconductor. When current flows in the semiconductor, enough electrons are moved from valence to conduction band to provide a population inversion. Light traveling through the semiconductor with the population inversion is amplified by stimulated emission. On the other hand, when no electric current flows, electrons move back to the valence band. Now the light is absorbed by the low energy electrons, so the light intensity is diminished as it travels through the semiconductor. The substantial difference in light intensity when current is flowing and when current is not flowing provides the on and off signals. However, this scheme is limited by the time for generation and relaxation of excited states in the semiconductor.
A third method, a Mach-Zehnder modulator, provides another external modulation technique in which a light beam traveling in a waveguide is split into two paths and then recombined into a single path where the two beams interfere. A material is provided along one path that has a refractive index sensitive to applied voltage. The change in phase introduced by the changing voltage applied to the material provides for constructive or destructive interference where the signals recombine. Currently, however, 10-15V is needed to provide the phase shift, and it has been difficult to provide high frequency signals at a high voltage to drive the phase modulator.
An alternative approach to increase the amount of data that can be transmitted through an optical fiber is Dense Wave Division Multiplexing (DWDM), in which many individual signals, each with a slightly different wavelength, are transmitted through a single optical fiber at one time. Each of the dozens of signals in the fiber runs at the 40 GHz data rate, providing a substantially higher overall data rate. While DWDM increases the data rate provided by a fiber, the equipment cost for transmission capacity is higher for DWDM than for faster modulators. Also, errors may be introduced into the data as a result of a process known as four wave mixing, in which photons of different wavelengths in a fiber combine, so data is lost in two channels in the fiber. Two other photons are generated at different wavelengths, and these may contribute to noise and errors in other channels in the fiber. Thus, faster modulation for each wavelength is desirable.
Two additional techniques to greatly increase modulation frequency and increase the data rate for transmission in a fiber have been proposed in commonly assigned U.S. Pat. No. 5,768,002 to K. A. Puzey, and in a paper xe2x80x9cMagneto-Optical Modulator for Superconducting Digital Output Interface,xe2x80x9d by Roman Sobolewski, et al, given at the Applied Superconducting Conference held Sep. 17-22, 2000 (xe2x80x9cthe Sobolewski paperxe2x80x9d). Superconductors allow low voltage high speed current switching.
The Puzey technique rapidly switches a superconducting film between superconducting and non-superconducting states and takes advantage of the difference in optical properties of the material in the two states. In the superconducting state, more far-infrared light is reflected from the material, while in the non-superconducting state, more is transmitted. Continuous wave far-infrared light is modulated by an electrical signal provided to such a superconducting film. After modulation of this far-infrared light, the signal is then parametrically converted to a shorter wavelength in the near-infrared range for transmission in a standard optical fiber. Well known frequency up-conversion nonlinear optics are used for the conversion.
The technique described in the Sobolewski paper stimulates magneto-optic material 10, such as europium monochalcogenides (EuS, EuTe, EuO, and EuSe) by providing magnetic field 12 from current pulse 14 in adjacent superconducting signal electrode 16 driven by a Josephson junction, as shown in FIGS. 1a, 1b. Continuous light wave 18 is coupled into magneto-optic material 10 through fiber optic input 19a and exits through fiber optic output 19b. Portion of light wave 18 traveling in magneto-optical material 10 in magnetic field 12 has its polarization rotated, a property known as the Faraday effect. An interferometer is used to provide pulses of light based on this rotation of the polarization. Because the excitation of magneto-optical materials occurs in a time measured in pico-seconds, as shown in FIG. 2a from a paper, xe2x80x9cFemtosecond Faraday rotation in spin-engineered heterostructures,xe2x80x9d by J. J. Baumberg, et al, J. Appl. Phys. 75 (10), May 15, 1994 (xe2x80x9cthe Baumberg paperxe2x80x9d), early investigators recognized that such magneto-optical microstriplines might provide a way to modulate signals in the THz (trillion bits per second) range, about two orders of magnitude higher than present modulation.
The curves in the Baumberg paper, however, show a problem with the slow relaxation from the excited state that limits the overall transition time. The relaxation time of magneto-optical materials from their excited state back to ground state can be much longer than the time for excitation, as also shown in FIG. 2a from the Baumberg paper. Thus, there is a very fast excitation rate, on the order of one picosecond, for Faraday rotation in an applied magnetic field for a heterostructure. There is also a slow exponential relaxation rate extending over 250 ps. The slow relaxation limits the speed at which a magneto-optical material can operate as an optical modulator. No way to avoid the slow relaxation has been demonstrated. This lengthy relaxation time substantially limits the speed of operation of such devices as compared to the promise of the much more rapid excitation.
Similarly, in a paper, xe2x80x9cUltrafast magneto-optic sampling of picosecond current pulses, xe2x80x9d by A. Y. Elazzabi and M. R. Freeman, Appl. Phys. Lett. 68 (25) Jun. 17, 1996, data is presented showing current pulses having a rise time of 15 ps and an exponential fall time of 250 ps obtained by triggering a photoconductive switch with an ultrashort laser pulse. The current pulse is used to change the refractive index of a Bi-substituted yttrium-iron-garnet ferromagnetic film, and this causes a rotation in the plane of polarization of polarized light due to the magneto-optic Kerr or Faraday effect in the ferromagnetic film. The technique allowed a bandwidth of 82 GHz to be achieved.
In a paper, xe2x80x9cFrequency-dependent Faraday rotation in CdMnTe,xe2x80x9d by M. A. Butler, et al, Appl. Phys. Lett. 49 (17), Oct. 27, 1986, data showing a large optical Faraday rotation as a function of the frequency of an applied magnetic field is presented for two compositions of the dilute magnetic semiconductor, CdMnTe. Faraday rotation was observed at frequencies up to about 5 GHz in small magnetic fields. This material has a very high response to magnetic excitation (a high Verdet constant), and it has a relaxation time constant on the order of 100 ps.
Although a number of authors have suggested advantages to modulating light based on magneto-optical materials, none suggest a scheme to overcome the limitation on data rate provided by the slow relaxation of the magneto-optical materials. Thus, a better system for more rapidly converting an electrical signal to an optical signal is needed, and this solution is provided by the following invention.
It is therefore an object of the present invention to provide a scheme for rapidly modulating an optical signal;
It is a further object of the present invention to provide a scheme for rapidly rotating polarization of an optical signal while eliminating a slow relaxation of the polarization;
It is a further object of the present invention to provide that delay associated with relaxation from an excited state of the magneto-optical material does not affect data rate;
It is a further object of the present invention to provide rapid changes between two states in a magneto-optical material to provide the rapid modulation of the optical signal traveling through the magneto-optical material;
It is a further object of the present invention to provide stimulations to the magneto-optical material for the transition in each direction so relaxation time does not limit the data rate;
It is a feature of the present invention that two rotations of the plane of polarization of light are provided by stimulating the magneto-optical material twice;
It is a feature of the present invention that two rotations of the plane of polarization of light are provided by stimulating the magneto-optical material with a single current pulse crossing the material twice;
It is a feature of the present invention that a superconductor is used to couple a current pulse which induces a magnetic pulse stimulation to the magneto-optical material;
It is an advantage of the present invention that a linear combination of two high speed stimulations to the magneto-optical material rotates the polarization back to its original direction while the effect of the two slow relaxation times on the light cancel each other out; and
It is an advantage of the present invention that the optical modulating is at a much higher data rate than is otherwise achievable.
These and other objects, features, and advantages of the invention are accomplished by a method of generating an electromagnetic pulse comprising the step of directing incident radiation through a material. The radiation has a first parameter. A first stimulation is provided to a first region of the material to excite a first population of the material into a first excited state. The material has a time for excitation and a time for relaxation after said excitation. From the incident radiation a pulse of electromagnetic radiation is generated in the material in which the pulse is shorter in time than time for the excitation plus the time for relaxation of the material.
Electromagnetic radiation traveling through the first region stimulated in the first excited state and radiation traveling through the second region stimulated in the second excited state are combined. The first parameter of the combined radiation comprises a linear combination of the first amount and the second amount. The linear combination of the first amount and the second amount provides the first parameter with a value approximately equivalent to that of the initial radiation. The first region of the material can be in a first leg of a Mach-Zehnder interferometer and the second region of the material in a second leg of the Mach-Zehnder interferometer. Alternatively, the first region of the material can be in line with the second region of the material, wherein radiation traveling through the first region stimulated in the first excited state also travels through the second region stimulated in the second excited state.
The second amount is set to be about equal and opposite the first amount so that slow relaxations cancel each other out.
The pulse includes a first part and a second part. The first part includes radiation traveling through the first region stimulated in the first excited state but not through the second region stimulated in the second excited state. The second part includes radiation traveling through the first region stimulated in the first excited state and radiation traveling through the second region stimulated in the second excited state.
The first parameter can be polarization, wherein the incident radiation has an incident polarization. The incident radiation traveling through the first region stimulated in the first excited state has the incident polarization rotated a first amount in a first direction. The radiation traveling through the second region stimulated in the second excited state has its polarization rotated a second amount in a second direction. The second part of the pulse includes radiation having a polarization rotated back to that of the incident radiation.
Another aspect of the invention is a device comprising an electrical conductor and a magneto-optical material. The electrical conductor crosses the magneto-optical material in a first location and in a second location. The electrical conductor is positioned to provide a first current pulse stimulating a first excitation of a first population of said magneto-optical material at the first location. The electrical conductor is positioned to provide a second current pulse stimulating a second excitation of a second population of the magneto-optical material at the second location. The electrical conductor and the magneto-optical material are configured to provide the second excitation of the second population to be opposite the first excitation of the first population.
Another aspect of the invention is a method of making an optical signal comprising the step of providing a material. Incident radiation is directed at the material. The incident radiation includes a first parameter having an initial value. The incident radiation also includes a first segment of the wave. A first stimulation is provided to the material to provide a first change to the first parameter in the first segment of the wave. A second stimulation is provided to the material to provide a second change to the first parameter in the first segment of the wave.
Another aspect of the invention is a circuit comprising a first superconductor, a second superconductor, and a source of a high frequency signal. The source has a frequency sufficient to provide resistance in the first superconductors and in the second superconductor for dividing the signal according to the resistance in each superconductor.